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Tunnel Question and Answers

New Questions and Answers added 06/26/08

Construction | Design | Geotechnical/Geological | General | Systems/Fire Safety | Ventilation


Question 1: Would you describe to me, in a short way, the different stages of building a tunnel?.

Answer: The following is a list the major aspects for consideration when constructing a tunnel.

  1. Determine the alignment and clearances (i.e. the size of the bore)
  2. Conduct a geotechnical investigation to determine the types of materials that the tunnel will be passing through.
  3. Determine the probable construction method (e.g., TBM, drill & shoot)
  4. Determine drainage and lining requirements.
  5. Determine appurtenances such as safety provisions, service buildings, ventilation, lighting, power supply, signal and communication systems, fire protection
  6. Establish operation and maintenance procedures.

Each of the listed items consists of many details and sub-processes that are interrelated and work together to result in a successful tunnel project. For detailed information I recommend that you see "Tunnel Engineering Handbook" by John O. Bickel and T. R. Kuesel.

Question 2: Can you provide sources of information to look at and a general outline so I can tackle the following.... Describe appropriate tunnelling method detailing resources and plant used for these cases..

  1. The construction of a road tunnel through fresh igneous rock. The tunnel forms part of a major highways scheme linking sections of motorway which have been constructed using normal cut and fill techniques.
  2. An 8km long tunnel extension to an existing underground system through chalk/chalk marl containing several minor fault zones. Access to the tunnelling operations is to be via a large 30m dia shaft.
  3. A 2m dia water main through mixed ground conditions including firm to stiff fissured clay and dense to medium dense gravely sand. Access to the site is facilitated by a shaft.
  4. A 1.5m dia 500m long intercept sewer which is required to run beneath an existing road and railway line through firm clay containing pockets of loose sand and compressed peat.

Answer: These are some very detailed questions and the answers will depend on various circumstances which have not been elaborated here and would, in any case, require some time to review. In general, TBM methods are best suited to soft ground tunneling and drill and blast to hard rock tunneling. For intermediate ground conditions, a road header, with or without a shield, or tunnel jacking can be used depending on the consequences of ground movement. For more information we refer you to the Tunnel Engineering Handbook by John O. Bickel and T. R. Kuesel.

Question 3: What are the methods of "keeping a placing barge in steady position on the sea" with high currency, during the procedure of "submerging" huge concrete tunnel sections into 55 meter below the sea level, placing them onto the seabed?

Answer: Most operations rely on tug boats to hold the barges in place when setting tunnel section units as well as timing the placement during a slack tide (a period in which the tide reverses direction). If the location in which you are placing the tunnel section units does not have a slack tide, it is necessary to look into using some sort of anchor lines or spuds to hold the barges in place. Another option is to use winch lines that are anchored to a stationary location.

For more detailed information on this complex construction activity, we suggest that you contact a contractor with experience in barging/submerging tunnel section units.

Question 4: Do you have cost information in relation to hard rock tunnel boring machines in the 3-5 meter diameter range? - preferably expressed in costs per meter or costs per kilometer

Answer: There are many factors that effect the cost (per tunnel length) to drive a tunnel through hard rock using a TBM, including fixed costs such as equipment, plant and cutter costs, and variable costs such as labor, equipment operation and maintenance, power, supplies, etc. However, for a 3 to 5 meter diameter TBM in hard rock (compressive strength >25,000 psi or ~172 MPa) the cost, for excavation only, would be in the range of $2500 to $3000 (US$) per linear meter of tunnel. Note that this does not include any installed support such as rock bolts or steel sets. It also does not include appurtenances such as lining, drainage, lighting, ventilation, etc. For more information, refer to "Tunnel Engineering Handbook" by Bickel and Kuesel. Chapter 10 deals with Tunnel Boring Machines and has information concerning costs and cost estimating.

Question 5: I'd like to know about horizontal grouting in soft soil tunneling. How is it performed? What type of grout is recommended for a silty clayey soil?

Answer: Grouting as a means of stabilizing soils has more often been used in the U.S. in shaft sinking and to repair collapses than as a routine method because it is an expensive and time consuming process that is not perfectly reliable even when very great care is exercised. The appropriate type of grout is dependent the purpose of the grout (increasing the strength or watertightness) and the permeability of the soil. Generally, for a silty clayey soils (i.e. ML or ML-CL in the USCS Classification System) with permeabilities in the range of 10(-5) to 10(-7) ft/min, the most appropriate grouts would be resin grouts, Acrylamide for watertightening applications or Phenolic for strengthening applications. These grouts rely on the in-situ polymerization of liquid organic solutions to form a solid product. The viscosity of this class of grouts is very low and remains constant until the grout has set, so very high penetrations can be achieved.

The methods in common usage are stage, series, circuit and packer grouting. In stage grouting, the holes are drilled and grouted successively deeper, the hole being washed out between stages before the grout hardens. In series grouting, new holes are drilled from the surface for each successive deeper zone, the holes remaining full of grout after each step. In circuit grouting, a double pipe is used. The injection pipe is at the bottom of the hole, the top being closed by a packer. Grout is forced in under pressure and any not flowing into the formation returns via the outer annulus to the holding tank. In packer grouting, part of the hole is isolated for grouting by expanding packers above and below the zone of interest. Grouting is performed from the bottom up.

Question 6: Can you please discuss the various approaches in rock bolting applied around the world

Answer: Your question is very broad and therefore not easily answered. Since our website is designed to answer questions dealing with tunneling issues, we will attempt to answer your question, only as it pertains to tunneling.

In rock tunnels, there are several methods that can be used to provide temporary support of the opening, one of which is rock bolting. Rock bolts are used to provide support to fractured or jointed rock so that it behaves as an intact rock mass. Typically in tunneling through rock, rock bolt applications are selected based on: 1) previous experience in similar rock, (i.e. prescribed rock bolt length and spacing); 2) support of individual rock units (i.e. rock bolt design is a function of the computed rock load per bolt); or 3) full roof rock reinforcement, (i.e. a systematic approach based on the assumption that rock can be confined to become part of the structure supporting the opening).

Early applications of rock bolting in highway tunnels in the United States were to provide temporary tunnel excavation support during construction. Rock bolts now may be used as permanent support in certain types of rock if proper design assumptions are made. Permanent rock bolts must be protected from corrosion by completely filling the drill hole with grout and protecting the bolt head with shotcrete.

More information on rock bolting in tunnel design can be found in "Tunnel Engineering Handbook", Bickel and Kuesel. More information on rock bolting in general can be found in many references including Federal Highway Administration publications, "Rock Slopes", FHWA HI-99-007, 1998, and "Rockfall Hazard Mitigation Methods", FHWA SA-93-085, 1994 and the Transportation Research Board publication, "Landslides Investigation and Mitigation", Special Report 247, 1996.

Question 7: What are the methods of ground water control techniques that can be used in the construction an open-cut tunnel?.

Answer: Karl Terzaghi stated that "all the serious difficulties that may be encountered during the construction of an earth tunnel are directly or indirectly due to the percolation of water toward the tunnel." The control of water is certainly of critical importance in tunneling.

The methods of groundwater control in tunneling are dewatering, grouting, compressed air, freezing and special construction methods.

Dewatering is the simplest and cheapest method of controlling groundwater by pumping from wells. However, there may be undesirable side effects from consolidation of the soil subject to increased effective weight.

Grouting can be used to control groundwater and at the same time reduce surface settlements. However, it is normally an expensive and time consuming process and is not perfectly reliable even when great care is exercised.

Compressed air is most often used to stabilize the ground in tunnels constructed in permeable soils below the water table, where dewatering is impractical.

Freezing is more commonly used in shaft sinking than in tunneling, but the method is useful where nothing else will work, providing there is access to the ground surface over the alignment of the tunnel.

If the ground is reasonably uniform and there are no obstacles, a tunnel boring machine (TBM) to tunneling in pervious ground below the water table as its use can reduce labor costs, increase safety and shorten construction time.

For more detailed information on this subject see "Tunnel Engineering Handbook", Bickel & Kuesel.

Question 8: We are doing a research project on TBM design and construction. Do you know where we can find the best resources to learn about the different steps in the design and construction phases of tunnel boring? Also, are there any simulators of tunnel boring machines in which we can view online?

Answer: The best place to go for information on TBM design and construction is the manufactures. The TBM Exchange International has a website (http://www.tbmexchange.com/) with a lot of good information on tunnel boring machines and other tunneling technology. For a primer on TBMs we recommend Chapter 10 - Tunnel-Boring Machines, of "Tunnel Engineering Handbook" edited by John O. Bickel and T.R. Kuesel, 2nd Edition, 1995.

Question 9: Where do you foresee tunnel boring in the next 20 years? Can you also briefly explain step by step how a tbm machine works? (how the machine moves forward, where the spoil goes, etc.) and how this machine is mounted to the ground.

Answer: A tunnel boring machine generally is a device for excavating a tunnel in such a way that the material to be removed is disintegrated by the continuous rotation of a group of cutting tools thrust against the surface of the material at the working face. The cutting tools are mounted in an arrangement suitable to excavate a tunnel of the required diameter.

The machine body is mounted immediately behind the cutting head and remains stationary while the cutting head excavates. The machine body contains mechanisms to provide the required torque and thrust to the cutting head as excavation progresses and maintain its stationary position or move itself forward, as required.

The tunnel boring machine is held in place, advanced and steered using hydraulically actuated main legs with gripper pads on the ends and located immediately behind the cutting head. The gripper pads consist of curved shoes with conical button inserts that are thrust against the tunnel wall during excavation to hold the machine in position. In addition, there are smaller support legs at the rear of the machine that are used only during the advance cycle. During excavation, the cutting head is thrust forward by means of four hydraulic jacks until the cutting head reaches the end of its stroke. During the advance cycle, the main legs are retracted and the machine is supported by the rear support legs and the cutting head. The main legs are advanced as a unit as the thrust jacks are retracted. The main legs again clamp the boring machine into position, the rear legs are retracted, and the next excavation cycle begins.

Muck is usually removed by a number of buckets on the cutting head and dropped onto a conveyor belt system to the back of the tunnel boring machine where it discharges into another transportation system (mucking cars or another conveyor system).

Question 10: What is the different of "bottom-up" and "top-down" method of tunnel construction? And what the special advantages and disadvantage of each?.

Answer: When you refer to the terms "bottom-up" and "top-down" construction, we assume that you are referring to the order in which material is removed from a tunnel cross section, i.e. "invert-to-crown" or "crown to invert", respectively.

Because of the excavation equipment available today, full-face excavation is the preferred excavation method. However, in cases where poorer rock is expected to be encountered, the use of staged excavation, supporting the excavation as it progresses, may be required. The decision of whether a bottom-up or top-down procedure should be used depends primarily on where the poorest quality rock is anticipated to be encountered. If shear zones are expected to be encountered then early support of the crown becomes most important. When the rock is generally of poor quality throughout the tunnel cross section, the engineer may choose to begin excavation at the tunnel invert and at the haunches and then provide support as the excavation proceeds upward and inward.

Another excavation method that has been successfully used in earlier years was to begin with a center heading and then ring drill the periphery of the tunnel. This is not used much anymore because of the larger and better drilling equipment now available.

Question 11: I would like to know the most likely method that will be used to construct a 700-800m long twin road tunnel (16mx10m each) of which approximately one third will be in soft ground and the remaining in rock/mixed ground.

Answer: Soft ground is typically excavated using a shield, with or without compressed air depending on the groundwater conditions. Rock is typically excavated either by drilling and blasting or with the use of a tunnel boring machine (TBM). However, if the final cross section shape of the tunnel is not round, as in this case, a TBM will not be efficient and drilling and blasting will more likely be the excavation method of choice.

The problem come when the tunneling operation encounters a transition between soft ground and rock or mixed ground conditions. The choice of excavation method will likely be largely driven by the variability the ground, i.e. how often is a transition likely to be encountered. If the zones of soft ground, rock and mixed ground are extensive and well defined, the contractor may choose to use different excavation methods in each zone. If ground conditions are variable the contractor will likely select the shield excavation method for the whole tunnel and modify the excavation procedure accordingly as rock and mixed ground conditions are encountered.

Question 12: What types of boring and reinforcement technologies are used when building high elevation tunnels (Eisenhower Memorial Tunnels) versus sub-aqueous tunnels (Hampton Roads Bridge Tunnel)? What kinds of future technologies do you foresee coming into common usage in either of the above situations?.

Answer: High elevation, alpine tunnels such as the Eisenhower Tunnel are usually constructed using conventional drill and blast methods or a tunnel boring machine (TBM) depending on factors such as length, final cross sectional shape and rock quality variability. Most drill and blast tunnels constructed today are supported using the New Austrian Tunneling Method, a method where the surrounding rock or soil formations are integrated into the overall ring-like support structure. Thus the supporting formations themselves become a part of the supporting structure. This method was developed by the Austrians in the later part of the twentieth century and has been used extensively in rock tunnels. Recently the method has been used in more in soft ground conditions.

Sub-aqueous tunnels are typically constructed by the immersed sunken tube method with either steel or concrete segments constructed in a ship yard and then barged to the site, sunken in place and connected together. There are a few sub-aqueous tunnels that have been constructed using a TBM such as the Chunnel between England and France.

The Hampton Roads tunnels were built by the immersed sunken tube method, comprised of shipyard-built prefabricated tunnel elements each about 300 feet long, placed by lay-barges and joined together in a trench dredged in the bottom of the harbor, and backfilled over with earth.

The Eisenhower Memorial Tunnel and its parallel twin, the Edwin C. Johnson Memorial Tunnel, were driven using conventional drill and blast methods. Geologic studies show that bedrock in the tunnel areas consist of 75 percent granite and 25 percent gneiss and schist. There are local dikes of augite diorite. There are faults and shear zones as well as solid bedrock. In the pilot bore, 26.5 percent of its length was in self-supporting rock; 73.5 percent required supports in varying degrees, and the total footage of what is commonly termed "bad" rock was 820 feet.

Question 13: Could you please describe the process of constructing a tunnel step-by-step, and the time it would take to complete each step. Can any of the steps be overlapped? How long would it take to complete a tunnel of about 150 ft, if there was piping that had to be relayed to avoid interfering with the path of the tunnel?.

Answer: The answer to this question in terms of time requirements depends on many factors such as, ground type (e.g. soil or rock), location (e.g. urban or mountains), construction method (e.g. TBM or drill/blast), tunnel size and shape, and the end-purpose of the tunnel (highway, rail, storm drain), and cannot be answered without considerable more project specific information.

A successful tunnel construction process will always begin with proper planning including a comprehensive geotechnical investigation. A step-by-step process for tunnel construction would include: prepare and stabilize one or both portals; excavate the tunnel (one or two directions); stabilize the tunnel interior (sets, rock bolts, etc.); install water proofing and drainage systems as required; construct lining (if required); and install appurtenances (e.g. lighting, ventilation, fire/life systems). On longer tunnels, two or more of these steps can be going on simultaneously at different locations along the tunnel in a "construction train" process.

Question 14: We are preparing contract documents for a tunnel at the Caldecott Tunnel Project. We would like to know whether itemized contract with each item as separate bid item or the per mile length of tunnel as a bid item is preferred by the industry. What are the advantages and disadvantages of each method?.

Answer: It has been common practice to prepare bid documents with pay items for each element to be incorporated into the tunnel project. The advantage is that the contractor can make a better estimate of every quantity of materials he/she must order and can estimate the effort to complete every item. It also helps the designer to prepare a better Engineers Estimate for the same reason. In addition, a common practice used on recent tunnel projects include provisions to keep the wining bid offer into an escrow file. This escrow file contains all assumptions made by the contractor while preparing his bid offer.

Question 15: Can you describe the use of Lattice Girders and Starter Bars in Tunnels constructed in rock. Do you have pictures and/or details for:

  1. Lattice Girders application.
  2. Spiles (timber or steel)
  3. Spacer Bars.

Answer: Steel sets, lattice or otherwise, are not commonly used in modern highway tunnel construction. It is more common to use either TBM or NATM methods. One exception to this was in Wyoming where unfaced rock tunnels were rehabilitated using steel lattice sets in conjunction with shotcrete as lagging.

For more information on lattice girders and associated members used in tunnel construction (including photographs and drawings) a good source is manufacturer's websites, such as http://www.americancommercial.com/.

Question 16: What is the importance of preliminary investigation in tunnel construction? What consists of preliminary investigations?.

Answer: In tunnel design and construction, the rock or soil through which the tunnel is excavated is as much a construction material as the concrete and steel used in other phases of the work. Explorations for tunnels are made to help determine the feasibility, safety, design and economics of a project.

The steps of a complete exploration program may include: Search of available literature and records; aerial photography study; surface geological reconnaissance; geophysical survey; exploratory borings; test pits, drifts and shafts; in situ testing; laboratory studies; full-scale model testing; actual construction; and post-construction monitoring and performance. The first three steps listed above could be considered to make up the "preliminary investigation".

Question 17: What are the pros and cons of using spray-on waterproof over membrane systems for tunnels constructed using shotcrete linings? Has there been any reported problems on either?.

Answer: When used under the proper conditions and by experienced contractors, the spray-on waterproofing can be less expensive than a membrane system. We have had experience with a spray-on lining used in the Wolf Creek Tunnel in southern Colorado. In this case, there were some problems with coverage that required several applications, whereas the contractor was anticipating coverage with a single application. This was also the contractor's first experience with spray-on waterproofing. The Wolf Creek Tunnel was advanced through fairly competent rock using NATM, including an initial shotcrete lining. The coverage problem was attributable to the very rough surface left from the drill-and-blast operation, which persisted despite the shotcrete layer.

Question 18: What is definition of slope stability analysis? What is the situation and propose methods of construction that is cost effective and feasible with minimum impact on the existing structures in the vicinity and minimum hazards to safety, health and environment of the surrounding when we use the cut and cover method?.

Answer: In general, slope stability analysis is an analytical method of determining the factor of safety of an earth or rock slope against failure. Stability of slopes is usually analyzed by methods of limit equilibrium. Factor of safety is defined as the ratio of the shear strength divided by the shear stress required for equilibrium of the slope. In the case of cut-and-cover tunnels, slope stability can be analyzed similar to retaining wall design where the walls of the tunnel are designed to counteract loads imposed by earth, hydrostatic, surcharge and seismic loadings.

The cut-and-cover tunneling method is usually economical for tunnel depths up to 20 meters. The tunnel is designed as a rigid frame box structure. In limited space conditions the tunnels walls can be constructed using neat line excavation utilizing sheet pile, soldier pile and lagging, drilled shaft or slurry wall construction. Support for wall construction can be provided by cantilever, internal bracing or ground anchors. Other considerations include groundwater control, control of floor heaving, tunnel waterproofing, and design of the tunnel top to resist future anticipated loadings.

Question 19: I have to write a school report on all tunnel construction techniques. Can you help me?.

Answer: There are several methods of tunnel construction, the selection of which will depend on considerations such as the material being tunneled through (e.g. soil, rock, mixed ground), depth of tunnel below grade (i.e. cover), length of tunnel, etc. Common methods of tunnel construction include: drill and blast, with support being provided with either the Austrian Tunneling Method (NATM) or sets and bracing; shield tunneling; tunnel boring machines (TBMs); sunken tube tunnels for underwater crossings; and cut-and-cover tunnels. For detailed information on tunneling methods and other important considerations for tunnel design and construction see "Tunnel Engineering Handbook" by Bickel and Kuesel.

Question 20: I am doing a research about cut-cover tunnel, would you please tell me, when I build a cut-cover tunnel, which method is better: top-down method and bottom-down method.

Answer: The decision to use either bottom-up or top-down construction methods for a cut-and-cover tunnel depends two issues. The first is available right-of-way. The use bottom-up construction methods, the excavation is first constructed by laying back the side slopes that requires significant right-of-way or easement. Typically in urban applications right-of-way is limited requiring that top-down construction methods be used. The second issue is ground water table. If the groundwater table is above the base of the tunnel excavation and dewatering is anticipated to be difficult, some top-down methods construction can control inflow of water into the excavation.

Question 21: What are the current methods and devices used for monitoring NATM tunnel stability?.

Answer: The stability of tunnels during construction is typically monitored using electronic distance measuring devices such as the "Geodimeter" or the "Tellurometer". This type of instrumentation is marketed by several manufacturers. Specific information can best be obtained by searching the internet using the device names as keywords.

Question 22: Could you please explain:

  1. forepoling method
  2. cantilever car dump method


  1. Forepoling is a method of advancing a mine working or tunnel in loose, caving, or running ground, such as water bearing sand or silt, by driving sharp-pointed poles, timbers, sections of steel, or slabs into the ground ahead of, or simultaneously with, the excavation. Forepoling is generally employed under the following conditions: the existence of shallow overburden above the tunnel; the need to restrict ground subsurface settlement; and poor ground conditions.

    There are three different methods of forepoling commonly used in tunneling:

    • Sub-horizontal jet grouting - Jet grouting is used in especially difficult conditions where both weak soils and thin overburden occur. Jet grouting is normally used in sandy or gravel formations.

    • Spiling - The spiling method consists of drilled steel pipes, grout inside the pipes and in voids outside, treated ground by permeation or fracture grouting and steel arch supports. Injection is the essential part of this system in forming the arch-like structure. Grouting is done to strengthen weak and loose soils, to decrease permeability, to reduce subsidence effects caused by dewatering and to stabilize sandy soils.

    • Pipe roof - Forepoling in the pipe roof method is formed in a crown of a tunnel by installing a series of large diameter steel or concrete tubes in an arch or a ring. This is done when overburden is especially thin. Typical installation methods are by pipe jacking and other micro tunneling methods. Pipe roof method pipes are designed to carry longitudinal loads only. They are typically made from open shafts and can be driven parallel to the tunnel axis.

  2. The cantilever dump method is a means of transporting spoils from a tunneling operation from the excavation face, through and out of the tunnel. In this method, cars are automatically and continuously transporting spoils out of the tunnel were they automatically tip to the side to expel the spoils to a spoils pile or other spoils transportation operation. It is often used as part of the Sequential Excavation Method (SEM). This method is based upon excavating the tunnel face either in whole or via pilot drives using backhoes or road header equipment. The excavated face is given a temporary lining of shotcrete, reinforced with steel ribs or reinforcing to minimize the relaxation of the in-situ stresses in the soil. The success of this method is dependent on minimizing the time between excavation and lining procedures. The cantilever dump method can be an efficient method for transporting spoils from the excavated face thus facilitating the SEM method. Another method commonly used to transport spoils with the SEM methods is a continuous conveyor system.

Question 23: What are the geological factors that govern the amount of overbreak in case of tunnels?.

Answer: Overbreak can be controlled in tunnel blasting, as in general rock slope blasting, by using controlled blasting techniques such as pre-splitting, trim blasting and line drilling which work best in massive rock formations. In geologically complicated material a simple crack does not form between adjacent blast holes. There is a broken shatter zone that forms that serves to act as protection for the final rock face from the effects of radial cracks emanating from the production blast holes.

When rock has numerous joints between blast holes and those joints intersect the face at less than a 15 degrees angle, it will be impossible to form a good smooth face with control blasting techniques. In fact, for the wall to have a reasonable degree of overbreak, the joints must intersect the face at greater than a 30 degree angle. Anything less will cause fractures to intersect the jointing planes having large pieces of material fall out from the face during the excavation process. Overbreak and also be caused by a poorly design blast plan that has the pre-split holes too close or overloaded. Close joint spacing can also interfere with a pre-split line. When this happens the solution is to reduce the hole spacing and powder load, make the burden larger than the hole spacing and/or detonate the pre-split holes simultaneously.

In less massive rock formations, the skill of the excavator operator is extremely critical. Some machines can exert considerable thrust, thereby digging into an unblasted wall severely damaging the final contour. Other geologic factors which affect the outcome of control blasting techniques are soft seams or mud seams. If the rock face is intersected by numerous mud seams it is difficult to produce good results.

Question 24: When we are using double shield TBM, can we identify the rock class? Please suggest how to calculate RMR.

Answer: By "rock class" we assume you mean Rock Mass Classification. There are 13 parameters that are included in a detailed rock description that describe the rock material, discontinuities, infilling, fractures or joints, and groundwater. These parameters can be evaluated during the exploration phase as well as during tunneling by mapping the tunnel walls behind the TBM and before the temporary and permanent facings are applied.

The Rock Mass Rating (RMR) is a method of quantifying the rock mass classification using numbers assigned to the 13 parameters mentioned above. The two most widely used rock classification systems were developed by the Council for Scientific and Industrial Research (CSIR) in South Africa (referred to as the Q-rating system) and the Norwegian Geotechnical Institute (NGI) (referred to as the Geomechanics system). The CSIR classification system is the most widely used procedure in the U.S. The CSIR method is fully described in FHWA GEC #5, "Evaluation of Soil and Rock Properties" FHWA IF-02-034 which can be downloaded from the FHWA website, http://www.fhwa.dot.gov/engineering/geotech/.

Question 25: What is the work sequence in tunneling project using road headers?.

Answer: The work sequence is not greatly different than standard drill-and-blast or shield tunneling. It is really a question of efficiency and selecting the best tunneling method for the given ground conditions. Road headers are typically used in soft rock conditions. Mixed ground conditions can be problematic with road headers. One advantage of road headers is that excavation and mucking can be conducted concurrently. , Ground support is usually installed shortly behind the road header and my consist of bolts, shotcrete and/or steel sets depending on the ground conditions. It is critical that the road header machine as well as the cutting head be design for the anticipated ground conditions including rock strength, rock variability and the grade of the tunnel (i.e. up or down grade). For more information, the US Corp of Engineers has a manual that you can find at http://www.usace.army.mil/usace-docs/eng-manuals/em1110-2-2901/c-5.pdf.

Question 26: I was wondering if you (or anyone) might be able to let me know the accuracy needed i.e.. First Order 1/100,000 or Order "B" if GPS is used, for tunneling construction controls for surveys. I have seen varying orders of accuracy used on highways depending on the type of highway. Also, I would like to know what methods are best (or preferred) for setting this control i.e.. GPS versus conventional traversing.

Do you know what methods are used to transfer the control from the surface to the bottom of the shaft and then how the control is prolonged up the tunnel? (plumb bobs dampened in oil drums on the reference line? Use of a Gyro-theodolite? Or other) Is it customary to drill holes (bore holes) at several places along the alignment for the tunnel surveyor to check into along the way? If so, what is a typical spacing?.

Answer: The follow survey specifications are recommended for major projects:

Triangulation - Second order Class I, closing error not to exceed 1:50,000
Vertical Control - Establish permanent benchmarks to the requirements of second order Class I
Primary Traverse - Second order Class I

The survey methods used to transfer working line and elevation underground and to set the laser beam of the tunnel construction control to line and grade should provide for this precision:

  1. Angular measurements to the nearest on second of arc
  2. Stationing to the nearest thousandth of a foot
  3. Benchmark elevation to the nearest thousandth of a foot

The precision of the target readings of the laser control system and tunnel ring measurements as performed after every shove should be in the range of one to two hundredths of a foot. The short time available for the performance of these measurements explains the lesser precision requirements. Primary control though triangulation for tunnels varies with the severity of the terrain through which the tunnel is being constructed (i.e. flat ground vs. mountainous). There are two common methods of transferring line and levels from the surface to the bottom of a shaft. One method uses transit sights where two work points are located on the working line at opposite edges of the shaft and after the theodolite is aligned, the line is extended down and across the bottom of the shaft. The other method is by means of steel wires supporting heavy weights hung in pails filled with oil that are set on the surface working line. An instrument in the shaft is then aligned with both wires so that it is on the working line and can be used to establish a work points at the bottom of the shaft.

When access through the portals or shafts is obstructed or impractical, such as for compressed air tunnel installations, it is advisable to bring control into the tunnel through survey holes sunk from the surface. Two holes at a distance of 200 feet on the working line are sufficient to transfer two work points from the surface into the tunnel and thereby establish a working line and stationing in the tunnel.

Question 27: Can you tell which type of TBM shall be most suitable for variable geological conditions. What is the approximate cost per meter of 300 mm thick segmental lining for a 6.1 m dia tunnel.

Answer: For variable ground conditions where you need to be ready to handle loose, soft or sheared ground, a double shield TBM may be required. Costs for the segmental lining in a tunnel can vary widely depending on factors such as tunnel length, access, transportation, groundwater conditions, grouting requirements, etc. The cost of the segmental lining is difficult to break out of the overall cost of the tunnel since this cost is usually not separated out in the bidding process. An very approximate number, for estimating purposes, for a 6 meter diameter tunnel excavated by TBM and lined with a segmental lining is $10,000/m.

Question 28: If the construction of a tunnel in an urban area is predicted to cause excessive settlement to structures above what measures can be taken to reduce this settlement?.

Answer: Traditionally, underpinning has been used to mitigate settlement of structures affected by ground settlement due to tunneling. However, traditional underpinning procedures can cause differential settlements on the same order of magnitude as those observed at the edge of the settlement trough. An underpinning system utilizing micropile technology may mitigate these settlements. Another approach to mitigating settlement due to tunneling that has gained acceptance in recent years is to employ some method of ground improvement. Ground improvement methods that have been used include permeation grouting and ground freezing. Ground freezing was successfully used on the Boston Artery Project where several tunnels were jacked under multiple rail lines that were in operation.

Question 29: When using compressed air in tunneling, what is the relationship between Particle Size Distribution curves (produced from sieve analysis of sand/soils) and the effectiveness of the compressed air? If the curves show a poor grading (lots of small and large particles - nothing in between) will air be lost through the sand and would grouting be a better option in this case?.

Answer: Compressed air is used in tunneling to counteract positive hydrostatic pressure below rivers or waterways or in porous soil below groundwater level. A small amount of air pressure can also be used in non-cohesive wet soil to dry up the face. Particle size distribution will affect the permeability of the soil and potentially the rate of air loss. Air loss can be mitigated by properly balancing the pressure with the hydrostatic pressure. The volume of air required is generally estimated to be 20 cfm/ft2 of face and must consider required ventilation for the workers. Air loss can also be mitigated by keeping the concrete lining as close to the face as practical.

Question 30: What's soil stratification in regards to tunneling?.

Answer: Soil stratification refers to the thickness and extent of the various classified soil types, and their physical or spatial relation to each other. In tunneling, this often includes rock (geological) stratification. One of the objectives of a geotechnical (and geological) investigation for a tunnel is to define the physical characteristics of the soil and rock materials through which the tunnel is to be driven and provide specific soil and rock design parameters. Some of the areas that require more detailed exploration are shear zones, water-bearing horizons, rock types with deep weathering potential, and topographic lows above the tunnel. Special attention is also paid to the materials and conditions at the portals.

Question 31: Comparing the NATM method with the TBM method in Tunnel Construction, what are the main advantages and disadvantages of each method?.

Answer: A TBM generally is a device for excavating a tunnel in such a way that the material to be removed is disintegrated by the continuous rotation of a group of cutting tools thrust against the surface of the material at the working face. A TBM works most efficiently in relatively homogeneous ground, (all soft or all hard), because it is very difficult to change cutters during the tunneling process. For mixed ground, where the tunnel passes from soft to hard ground, or visa-versa, NATM is usually a more economical tunnel method.

NATM was pioneered by the Austrians in the later half of the twentieth century. The tunnel is sequentially excavated and supported, and the excavation sequences can be varied. In soft ground tunnels, initial ground support in the form of shotcrete, usually with lattice girders and some form of ground reinforcement, is installed as excavation proceeds, followed by installation of a final lining at a later date. In cases where soft ground conditions do not favor an open face with a short length of uncompleted lining immediately nest to it (flowing ground or ground with a short stand-up time), a ground arch does not develop. Unless such unstable conditions can be modified by dewatering, spilling, grouting, or other methods of ground improvement, closed-face shield tunneling, and not NATM should be considered.

Question 32: What is forepoling in tunneling.

Answer: Forepoling is a once-practiced method (rarely used today) for presupporting running ground in conventionally driven tunnel headings - ground comprised of cohesionless soils and/or very weak rock that are not self-supporting at the face. The method generally consists of driving wood or steel spiles ahead of the face at the tunnel periphery, trimming the back of the spile to just out by the next steel rib set. When the steel set is placed, it pins the hanging back end of the spile to the roof.

Forepoling can also refer to chemical grout treated ground (injected in periphery holes drilled ahead of the face) and/or angled steel bar installations (grouted bolts) aimed at keying plates or blocks together prior to undermining. A couple of old, but good descriptions of conventional forepoling can be found in "Rock Tunneling with Steel Supports", Proctor and White, published by Commercial Shearing, and "Earth Tunneling with Steel Supports" by the same authors.

Most soft ground tunnels today are excavated with shield machines that can manage running ground either by rapid presupport at the face excavation with the shield canopy or by earth or slurry face pressure balancing methods. Forepoling is rarely used, though may find a niche in short tunnels in running ground where more elaborate tunneling equipment would be cost inefficient.

Question 33: What are the different methods of tunnel construction?.

Answer: The different methods of tunnel construction include: drill and blast; shield; cut and cover; tunnel boring machine (TBM); New Austrian Tunneling Method (NATM) also called Sequential Excavation Method (SEM); roadheader, sunken tube; and jacked. The more important question is how to determine which method is the most appropriate for a given project. Factors that affect the selection of tunnel construction method vary from geological to economical to sociological. Geological factors often effect economical considerations because ground conditions such as whether the tunnel passes through rock, soil or mixed ground can drive the construction method decision since some methods are more efficient in certain ground conditions. Shallow tunnels are often constructed using cut and cover techniques, especially in urban areas. Tunnels that cross bodies of water can be either bored or constructed using sunken tubes depending on the depth of the water, ground conditions and the length of the tunnel. Jacked tunnels have been used in urban areas to construct tunnels under active transportation facilities. An engineer experienced various methods of tunnel construction will be best equipped to help an owner/operator select the best method(s) of tunnel construction for their project. For more information on the tunnel construction methods listed above, a good reference is "Tunnel Engineering Handbook" by Bickel and Kuesel.

Question 34: Where can I find the requirements for lighting, ventilation, fire safety, etc for the construction of an exploratory tunnel?.

Answer: For an exploratory tunnel the requirements for lighting, ventilation, fire safety, etc. are those required by national and local codes, laws and regulations to protect the workers. In the US these requirements are given by The Office of Occupational Safety and Health Administration (OSHA).

Question 35: What are the precautions to be taken in constructing the tunnel in earthquake prone area having fault lines in the ground?.

Answer: Underground tunnels and facilities are generally less susceptible to damage from earthquake shaking than are above ground structures, though damage has been reported. Underground structures have relatively little mass, are embedded below ground, and are generally confined by the ground mass, and, therefore, are not as susceptible to damage from the inertial effect of earthquakes. Instead, seismic demand on underground structures results from kinematic interaction between the ground and the structure - with seismic loads characterized in terms of deformations and strains imposed on the structure by the surrounding ground mass.

Two types of ground deformations need to be considered: transient deformations and permanent deformations. Transient deformations result from the passage of compression and shear waves during an earthquake. During an earthquake, tunnels are subject to axial compression/extension and longitudinal bending from horizontally propagating waves, and to ovaling or racking for vertically propagating waves. Analyses involve identifying source locations, source magnitudes and wave propagation characteristics, tunnel site amplification, time histories for anticipated events, and incidence angles (from what direction the waves hit the structure).

Permanent deformations result from ground failures such as slope instability, lateral spreading, liquefaction, and fault rupture, to name a few. In settings subject to faulting, particular attention should be paid to the potential for renewed movement along the fault and/or the possibility for heightened water transport along fault surfaces. Extensive surface and subsurface mapping is required to determine the location and inclination of faults, general type of faults (normal, thrust, side lateral, etc.), historical movements along faults, magnitude and direction of movements, thickness of the failure zone (single failure plane, shear zone with numerous failure planes, thick fault gouge, etc.), presence of water (now or in the past), and relationship to local and regional structures (is the fault part of larger system, horst/graben sequence, etc.). Coupled with assessments of ground strain potential from nearby earthquake sources, fault displacement risks can be estimated and accounted for in tunnel lining design.

Question 36: What are the principles and criteria for hydrogeological studies of tunnel a route, if the tunnel route has an average of 300 m overburden and is located below ground water table?

How can one estimate the permeability of rock mass around the tunnel for evaluation of pressures on the lining, if permeability tests have not been performed inside boreholes?.

Water pressure tests are part of most rock exploration programs. Carefully done, they produce a great deal of useful information about subsurface conditions.

Before water pressure test data can be effectively used, the field results must be converted to a coefficient which gives a common base to the variables in the test (i.e darcy units, k).

In few rock types is the permeability more or less uniform throughout a rock mass. Generally, the value of k measured is an average of a wide range of values over the test section. Taken by itself, the test water loss often can give good approximations of the amount of water which will pass through a given stratum, providing, of course, that the test hole intersects rock and fractures which are typical of the rock mass overall. For other applications, such as grouting, the test water loss can be very misleading. For example, a test may give a permeability suggesting a fairly tight rock, while, in fact, most of the water may have been lost through one fracture. Consequently, when establishing grouting criteria, one must be careful about arbitrarily picking a value for k below which no grouting will be done.

Question 37: In case the soil is expansive and the tunnel lining has to be densely reinforced to resist the expansive forces, and it has been decided to place the concrete in situ, what type of concrete would you recommend? Why? What aspects of the concrete are important?.

Portland cement types I, II and III are equally acceptable. Selection of cement type often depends on compatibility with acceptable accelerators. Optimum setting times are obtained with the most compatible cement-accelerator combinations. Compatibility of a cement of particular manufacture and a particular accelerator requires testing. Where concrete may be exposed to sulfate attach, which is not uncommon in expansive soils, a satisfactory sulfate-resistant cement, such as Type II) is required. When reinforce concrete is densely reinforced, as in this case, consideration should be given to using "self consolidating concrete" (SCC). For more information on SCC see: http://www.nrmca.org/aboutconcrete/cips/37p.pdf (.pdf, 0.25 mb)

Question 38: Please explain the difference between "immediate support", "initial support" and "ancillary support.".

"Immediate Support" most commonly refers to temporary support measures taken to stabilize recently excavated ground prior to initial support installation. Such measures generally involve external support systems, and may include shields, jacks, timbers, etc., or perhaps spray on epoxies or light shotcrete coatings to limit water migration or weathering issues.

"Initial Support" commonly refers to the first support elements or system of support elements installed to stabilize the tunnel opening prior to final support placement. Initial support commonly consists of such elements as tensioned bolts, dowels, wire mesh, shotcrete, lattice girders, steel ribs, etc. These types of supports are placed at or near the advancing face shortly after excavation.

"Ancillary Support" refers to additional site-specific support measures implemented to address anomalous ground conditions, changes in tunnel geometry (e.g., intersections of tunnels or shafts), or support used to reinforce ground sections that may be subject to higher applied operational loads (e.g., overhead ventilation or power stations). Ancillary support may be nothing more than alterations in the initial support plan, implementing the same elements, or may involve more exotic ground control measures such as grout injection, cable bolting, etc.).

"Final Support" includes the finishing support element(s)of the tunnel - most commonly a final reinforced tunnel lining. In some cases, initial support may actually serve as the final support for the tunnel, with no additional lining required.

An example of these support systems might be found in a typical TBM drivage where shields at the face are used to temporarily support the ground (immediate support), initial support is installed at the back of the shields several to many meters from the face, and final tunnel lining is installed well back from the TBM. Ancillary support may be found at the portal, for instance, where additional support may be required to address low cover conditions.

Question 39: What equipment is used in immersed tube tunneling?.

http://www.ita-aites.org/cms/410.html provides good information about the construction of Immersed Tube Tunnels (ITT).


Question 1: What is the economical service life of tunnels?.

Answer: The information on economic service life of tunnels is very limited and not documented adequately; however, there are efforts underway to address this very issue. The first is a joint research initiative undertaken by Federal Highway Administration (FHWA) and Federal Transit Administration (FTA) that based lined the condition of tunnels in the United States, developed inspection procedures for different types of tunnels and created a data base that tunnel owners can use to track the condition of their respective tunnels. This effort has just been completed and in information will be posted on the internet at http://assetmanagement.transportation.org/.

Also, we would strongly recommend that anyone interested in evaluating the service life of tunnels to be familiar with the Life Cycle Cost initiatives that FHWA is promoting and the training offered. Granted, the information was directed towards bridge structures; however, the life cycle cost analysis/techniques should be the same, the only difference being the analysis is done on a tunnel rather than a bridge.

As more information on this topic becomes available, we will post it on the web site for use.

Question 2: Please provide information concerning the procedure for the design of underground large scale of rock excavation.

Answer: The design of large underground openings in rock is a complex, multi-phase process. The process begins with the characterization of the rock conditions in the vicinity of the planned excavation. Evaluation of surface geology, subsurface exploration, and laboratory testing of rock samples are some examples of exploration activities. This exploration needs to be accomplished prior to tunnel design to define the properties of the rock and structural characteristics of the formation.

After the exploratory program is completed and documented, design can proceed. Rock structure support and excavation techniques are then considered. There are many techniques available to excavate and support underground openings in rock, depending on the rock types, geologic conditions, and size and usage of the underground opening.

For more information, a very good reference text on the development of underground openings is "Tunnel Engineering Handbook", edited by John O. Bickel and T.R. Kuesel, Van Nostrand Reinhold Co., 1982.

Question 3: What are the general design consideration in designing a road tunnel? What are the typical hazards during construction and during operation and can you suggest possible solutions to any of the safety problems?

Answer: This is a very broad question. We refer you to a reference that can suggest solutions to various problems of this type, "Tunnel Engineering Handbook" by John O. Bickel and T. R. Kuesel. Internet searches may also get you information about specific tunnels for case study information. We welcome specific questions based on the information you gather.

Question 4: Could you please give me some reasons why precast concrete lining is better than other methods.

Answer: The advantages of using a precast concrete lining are similar for using precast concrete elements on other facilities: off-site fabrication, ease of replacement, factory control of fabrication and finishes and speed of erection. A precast concrete segmental lining is usually compatible with the TBM (tunnel boring machine) method of construction.

Question 5: What is the AASHTO design life for tunnel?

Answer: AASHTO does not specify design life for tunnels. The useful life of a tunnel is dependent on its serviceability which is dependent on the level of maintenance that is performed on the various components within the tunnel such as the lining, riding surface, and mechanical equipment. The high cost or impracticality of rerouting a tunnel or replacing it with a different type of facility makes it more reasonable to look at tunnels as facilities that require periodic maintenance expenditures for the various components with the expectation that the tunnel will have an indefinite service life as long as the maintenance is performed.

Tunnels vary in the type of material that the tunnel passes through, their construction method and details, and the type of components that exist as part of the tunnel structure. Therefore, maintenance requirements and expenditures can vary greatly. For example, an unlined tunnel through solid rock with little water seepage might require virtually no maintenance other than the routine rehabilitation of the roadway surface. On the other hand, tunnels with concrete linings, lighting and ventilation systems, and other safety and maintenance systems require require regular planned maintenance.

Question 6: Would you tell me where I can find information about the air capacity that is need in basic design of ventilation for railroad tunnels in rural area.

Answer: Thanks for contacting the FHWA Road Tunnel Virtual Team. As the name implies, the main focus of our team is with all matters related to Road Tunnels, nonetheless, basic information about ventilation of railroad tunnels is contained in the Tunnel Engineering Handbook, John O Bickel; T R Kuesel, Published by Van Nostrand Reinhold Co., NY. We encourage you to read this handbook for the information you are looking.

Question 7: The tunnel ventilation ceiling is built by cast in place concrete after the inner lining being casted. Where can I find information of air tightness of ventilation ceiling? Such as how to design the joint, or the test needed to be done prior to the full or partial ventilation test?

Answer: There is a short section on Cast-in-Place concrete ceilings in Chapter 17, Tunnel Finish (pgs. 457 - 461) in "Tunnel Engineering Handbook", by Bickel and Kuesel (2nd Edition, 1995). Sealing between concrete elements is achieved using neoprene pads or liquid lining and caulking. Chapter 19, Tunnel Ventilation (pgs. 483 - 563) of this same text has a sections on Pressure Evaluations.

Question 8: Is there a limit in length for a road tunnel to require ventilation?

Answer: The problem of ventilation for road tunnels short or long is a complex issue with several variables. The objectives of ventilation for road tunnels are, to dilute the carbon monoxide concentration to accepted levels for the traveling public, to effectively and efficiently manage the smoke and heat during a fire inside a tunnel. The design of a ventilation must consider in addition to length, elevation, grades, width, traffic volume, uni or bi-directional traffic, are pedestrians allowed, and fire conditions. It is suggested that you review the ventilation chapter in the Tunneling Handbook and the National Standard 502 National Fire Association for information on how to design your ventilation system, but keep in mind that your design must be approved by the authority having jurisdiction in your country.

Question 9: There appears to be a very wide opinion as to the lane widths and overall widths of tunnel (lining widths) from e.g. 2.8 m in France to 3.5 m or 12 foot in the US what is the standard used to set these widths and height for that matter. assume that speeds may be urban rather than "open highway"

Answer: In the US, the American Association of State Highway and Transportation Officials (AASHTO) has established guidelines (A Policy on Geometric Design of Highways and Streets, "Green Book") for the geometric design elements of highways. These guidelines are also applied to the tunnels.

Question 10: I am working on the ventilation system for an underground tunnel used for electric train transit only. I understand the "Tunnel Engineering Handbook" is an excellent reference. I have seen several references in an ASHRAE source for the "Subway Environmental Design Handbook Vol. 1" (DOT 1976) Is this a valid source to use today? If so, where would I find it? I've had difficulty finding it on the internet.

Answer: The Subway Environment Simulation (SES) computer program and Subway Environmental Design Handbook were developed in the early 1970's under sponsorship of the Urban Mass Transportation Administration to assist in the planning, design and construction of subway ventilation systems. The SES fulfilled an unmet need in the transit engineering community, and has been widely used in the design of new rail systems or line extensions in: Washington, DC., Atlanta, Buffalo, Baltimore, Dallas, Los Angeles, San Francisco, Montreal, Toronto, the Seattle Bus Tunnel, and in rail transit systems around the world. The SES provides tunnel designers with the tools to: properly size and locate ventilation shafts, evaluate tunnel geometry and fan size, optimize temperature, and model the effects of heat and smoke resulting from fires and other sources. The most recent enhancement is validation of the subroutine which describes the behavior of smoke in emergency conditions.

The currently available SES is written in FORTRAN 66 and the ASCII or EBCDIC Formats. Although the program was developed for a Univac 1108, it will run on any machine that meets the above specifications. It is available on 3 1/2 inch disk. The Volpe National Transportation Systems Center, Cambridge, MA., is in the process of modifying the SES to run on personal computers. The modified SES should be available by early 1997.

For further information, contact: Alison Thompson at the Volpe Center (617) 494-2108 E-Mail: THOMPSONA@VOLPE3.DOT.GOV.

Question 11: Have you heard of any published guidelines for frequency of expansion/contraction joints in cast-in-place concrete tunnels?

Answer: We are not aware of specific published guidelines for expansion and contraction joints spacing for tunnels. This is because the thermal effects are related to the depth of the tunnel, the length of the tunnel, the ventilation in the tunnel, environmental issues, location, etc.... For short tunnels (and the tunnel in question is one of them) the interior temperature is the same as the ambient temperature while the temperature of the exterior face of the walls or the slab is based on the soil cover over them. For shallow tunnels the top slab exterior face temperature could be very close to the ambient temperature while for deep cover or for the walls and the bottom slab the temperature on the outer face could be constant around 55F. Therefore, there is a thermal gradient between the inner face and the outer face of the tunnel walls and slabs that the designer should consider. Often no expansion joints are provided in tunnels, only contraction/construction joints 30 - 50 ft spacing are provided.

If expansion joints are needed, I suggest that careful attention to the waterproofing details should be provided because the joints are usually the source of water infiltration.

Question 12: What effect does vertical settlement (12 to 15") under storage piles (100 ft or more) resulting in lateral soil mevement (6 to 9") have on 12" micropiles, 3/8" wall 80 ksi, with 2 1/2" central 150 FY rebar? Does the pile yield and fail, or does the underlying soil limit and brace the pile?

Answer: This is a very specific design question that cannot be directly answered. You should consult the "Micropile Design and Construction Guidelines" FHWA-SA-97-070 for methods to assess micropile design capacities and fialure modes. It is available in .pdf format at http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm?TitleStart=M (our FHWA publications website).


Question 1: Is there existing data that shows tunneling though rock (fractured or otherwise) is less expensive than open cuts in mountainous areas. Is there a break point, in depth and length, where one method out paces another?

Answer: As you can imagine, there are no hard and fast rules concerning a cut off for open cut versus tunneling for either short tunnels or tunnels with minimal cover. In some recent highway tunnel projects the question was decided by desired aesthetics rather than economic or technical considerations.

This question came up on the Hoover Dam Bypass project where a tunnel was planned but was deleted in favor of an open cut when the investigation indicated that the quality of the rock would make support of the over burden difficult.

The bottom line is that this is a case-by-case decision that will be pushed by technical, economic, aesthetic and political considerations.

Question 2: What type of training and education does it take to become a tunnel engineer?

Answer: We suggest that a "4-year" Bachelor of Science (B.S.) degree in some engineering discipline is a basic requirement. Suggested disciplines for a B.S. degree include civil engineering, mining engineering, or geotechnical/geologic engineering. We do not know of any engineering colleges in the US that offer an undergraduate degree specifically in tunnel engineering. Of course, you should take advantage of any specific courses offered to undergrads on tunnel engineering.

Depending on your goals, an advanced degree in geotechnical engineering can be a valuable credential as a tunnel engineer. We also strongly recommend professional licensure as a means to obtain your career goals.

As in life, experience is very important in becoming a competent tunnel engineer. Any opportunity to work in or around a tunnel construction project (at any level) would be invaluable. If you are an engineering student, try to find a summer job on a tunnel construction project. It will allow you to "put together" the engineering theory with field practices.

Based on our experience, the best tunnel engineers are individuals that have a strong educational foundation in civil engineering, a masters degree in geotechnical engineering, and 5 or more years experience in tunnel construction. We believe there is a strong demand for tunnel engineers, and wish you the best of luck in your pursuits. .

Question 3: Could you please give me information on road tunnel regarding the following:

  1. tunneling through a rock
  2. why are road tunnels constructed instead of open cuts?
  3. what are the advantages of road tunnels
  1. Tunneling through hard rock can be accomplished by either drill and blast excavation, or the use of a tunnel boring machine. Tunnel support in good rock can usually be accomplished with rock reinforcement and shotcrete. In poorer rock, steel ribs and a full concrete structural lining may be necessary. For further information we suggest the most recent edition of the Tunnel Engineering Handbook, edited by John O. Bickel/T.R. Kuesel.

  2. Road tunnels may be a better choice than a large rock cut if, in general, one or more of the following conditions exist:
    1. Rights-of-way are too narrow to allow a rock cut.
    2. Traffic considerations do not allow blasting a rock cut above an active highway.
    3. Some aspect of the surface environment (existing structures, natural features, etc.) needs to be protected.
    4. Aesthetic considerations favor a tunnel rather than a surface alternative.
    5. It is difficult or cost-prohibitive to deal with the large volume of rock excavation generated by an open rock cut.

  3. Some of the advantages of road tunnels are listed in the answer to b. above. Other advantages include isolation from the surface environment, better resistance to natural forces encountered in earthquakes, and the ability to create a road facility in an area were it is simply not possible to accomplish a surface solution.

Question 4: Is there a current expected risk factor for fatalities per distance drilled. I understand that tunneling has made large safety increases in the past decade. How has this figure changed in the past decades.

Answer: According to the Tunneling Engineering Handbook, in the past, tunnel accidents claimed one life for every half mile of tunnel constructed. This rate have been reduced greatly in part to better construction methods, materials and government safety provisions and regulations. However, based on OSHA (Occupational Safety and Health Administration) data of 1986, tunneling related accidents still occurred at more than twice the frequency for other above the ground construction jobs and three times for manufacturing industry. The Tunnel industry has achieved great strides toward a safer work place, but there are still more that can be done.

Question 5: What is the specific FHWA definition of a tunnel? At what point does a longer underpass become a tunnel? Is it simply a function of lighting, ventilation, or enclosed length. What is the criteria used to determine if a portion of roadway is considered a tunnel in conjunction with the NEPA process?

Answer: There is no accepted standard definition for a tunnel. According to AASHTO a short tunnel is one with a length portal to portal less than the safe stopping distance (SST) for the design speed, and a long tunnel is one with a length portal to portal greater than the SSD. A structure can be classified as a tunnel when the construction method used involved any tunneling construction technique. A long underpass may need to be designed as a tunnel to provide a safe environment to the traveling public if location, geometry or traffic conditions warrants special services like ventilation, lighting, and emergency systems.

Question 6: Would you tell me where I can search about concrete ribs.

Answer: We recommend that you consult with companies that supply tunnel support products such as: Camber Corporation
602 McKnight Park Drive
Pittsburgh, PA 15237

Question 7: To re-capture a sense of downtown in our small town, I am exploring the possibility of submerging the main road through town for about 1500 feet. Is there a way of getting a preliminary cost per linear foot so that I can think about the feasibility of such a project?

Answer: Road Tunnel costs depend on my factors and variables. For example, these factors and variables include, depth of tunnel, need for temporary support of excavation, geometric dimensions of the structure, materials to be used, need for safety and security systems, traffic volume, need for temporary structures, utility relocation, soil properties, location of watertable, loads to be supported by the tunnel, and mitigation of environmental impacts. Therefore, we can not answer your question with the limited information you have provided.

Question 8: Is a tunnel an underground structure? Is there a specific definition that uses the two synonymously


  • Loosely applied to any horizontal shaft but a tunnel must be open to the air at both ends; an adit is open to the air at one end, if it were continued completely through a hill, it would be a tunnel.
  • A large, underground structure used to store and transport wastewater, combined sewage or storm water during rain storms. Tunnels usually have higher storage capacities than basins and are capable of transporting their flows directly to a wastewater treatment plant. http://www.wadetrim.com/resources/glos.htm
  • A passageway through or under something, usually underground (especially one for trains or cars); "the tunnel reduced congestion at that intersection." http://www.cogsci.princeton.edu/cgi-bin/webwn
  • A tunnel used to carry a roadway through a mountain or under a river instead of around or over those obstacles. Tunnels can be very expensive to construct and maintain due to drilling and ventilation requirements. Generally, most tunnels are located in mountainous or hilly regions, and examples include the Fort Pitt Tunnel (Interstate 279) in Pittsburgh and the Waldo Tunnel (U.S. 101) north of San Francisco. http://www.aaroads.com/glossary.html

Question 9: What role do tunnels have in road traffic?

Answer: There are many roles tunnels are used in highway applications. For the most part tunnels are used to avoid an obstacle.

Several highway tunnels are utilized in mountainous terrain where building the roadway on the outside could be very difficult. The benefit of tunnels in mountainous environment will improve the roadway alignment and reduce the vertical grade. Other examples for tunnels are used for under shipping channels in lieu of a bridge. We also are beginning to see tunnels used in urban intersections to relieve congestion. Tunnels can be used for drainage structures usually under under heavily traveled roadways.

Question 10: Was there ever a concern about carbonation in road tunnel? If yes, what were the measures taken to overcome or prevent this problem?

Answer: Carbonation of Concrete in Road Tunnel.

The carbonation of concrete is always a concern for any concrete structure we design and build.

In the road tunnels harsh environment carbonation of concrete lining is a concern.

Carbonation can lead to accelerated deterioration of the concrete and induce corrosion of the reinforcing steel.

In the past the use of the following practices have provided good results.

  1. Use good design and construction details.
    1. use good cover over rebars
    2. vibrate, and vibrate and again vibrate the concrete
  2. Use high performance concrete.
  3. Use high performance reinforcing steel.
    1. epoxy rebars have been used
  4. Use waterproofing membrane.
    1. to prevent corrosion, keep water out

Question 11: What is a Roadheader?

Answer: The roadheader was invented in the United Kingdom about 30-40 years ago as a multipurpose mining and construction tool. It consists of a crawler base similar to a mini-excavator which hosts an articulated boom with one or more rotating drums on the end. These drums are mounted in line or perpendicular to the boom and feature an array of replaceable teeth which dig and fracture earth or rock as the drums spin. Other special function heads include jack-hammer like spikes, compression fracture micro-wheel heads like those of Tunnel Boring Machines but in miniature, a slicer head like a gigantic chain saw for dicing up rock, and the simple jaw-like buckets of traditional excavators. A debris collector in front of the crawler sweeps up the excavated material onto a conveyor belt which deposits it in a pile or bin behind the machine. Some models include special laser guidance systems affording auto-leveling, precise grading, and digging with millimeter precision. A typical roadheader under normal conditions can excavate tunnels its own width as fast as 20 feet an hour and needs only one or two people to operate it.


Question 12: What are the factors affecting economics of tunnel design construction?

Answer: There are many factors affecting the economics of a tunnel. A tunnel is composed of many elements. Every tunnel is different depending on where it is located, its use, method of construction, environmental and safety requirements. Tunnels need to be designed with a proper cross section to effectively and efficiently provide a safe passage for its users, some kind of lighting always needs to be provided, ventilation, security and safety requirements are always needed. A tunnel cost should be based on a life cycle analysis and not only on the initial construction cost.

Question 13: Would you please tell me the advantages and disadvantages about driven tunnel method and cut-and-cover tunnel method, and which method I should choose in different situation?

Answer: The selection of driven or cut-and-cover tunneling depends on many factors.

If the tunnel is at shallow depths, usually the cut-and-cover construction method will be favored over driven tunneling. For short urban tunnels, cut and cover is usually chosen instead of driven tunnel.

Sometimes because environmental constraints the use of cut-and-cover is not allowed, and the driven option is favored in those situations. Also, when a tunnel must cross a body of water, the driven or immersed tube method has more appeal than the cut-and-cover method.

The cut-and-cover, usually involves the application of temporary support of excavation. The trench created for the cut-and-cover tunnel imposes a loss of surface open space or streets on the community. When the tunnel will be built beneath obstructions or buildings that can not be removed, the driven method is favored.

The construction method selected should be evaluated for the specific site conditions.

Question 14: What is the the best waterproofing system for cut and cover tunnel especially for base slab?

Answer: The best waterproofing system is the one that will provide your tunnel with a very low or no water intrusion, that it is acceptable by the owner. How you achieve this depends in many factors since every project has its own challenges, specific constraints and requirements.

In general, a good design should employ a multiple layer protection system. You should start by using good quality concrete with very low permeability. Several waterproofing membranes systems that can be applied under the bottom slab and around the tunnel wall are available and you should consult with their representative to help you select the one that will suite your specific needs. Especial attention should be given to the quality of installation, inspection, protection of system during construction, to direct drainage away from the tunnel and to effectively collect and discharge drainage.

Question 15: How do you define "self supporting" ground in microtunneling. Does it exclude any or all spalling from the roof of the tunnel?

Answer: Since your question is related to trenchless technology, I recommend that you contact the North American Society for Trenchless Technology for assistance at the following address.

North American Society for Trenchless Technology
1655 N. Ft. Myer Drive, Suite 700
Arlington, VA 22209 USA
Phone: (703) 351-5252 Fax: (703) 739-6672

Question 16: I often hear people talk about the use of steel sets in the support of tunnels. Can you explain how they are used and exactly what are they? Also could you explain rock bolts and how are they used.

Answer: Steel sets can be defined as a "passive" system used in tunnels for ground support, usually consisting of I-beams for caps and H-beams for posts or wall plates. The term "passive" derives from the fact that steel sets and arches do not interact with the rock the way that roof bolts do.

A rock bolt is a steel bolt or cable secured into place in the roof or rib of a tunnel opening for the purpose of pinning layers of rock together. Rock bolts can be used to create an "active" rock and bolt arch for the support of excavation as in the New Austrian Tunneling Method.

Question 17: I want to know the rate of accumulation of water while constructing a tunnel at a considerable depth

Answer: There are many factors that control the amount of groundwater flow into a tunnel. The height of the water table above the tunnel determines the water pressure, and the permeability of the ground affects the possible flow quantity. The extend in depth and size of the aquifer in which the tunnel is located and its distance from a possible source of (river, lake, etc.) also affects possible flow recharge.

Question 18: I want to know about maximum allowable leakage in a metro tunnel that constructed by NATM method and will be lined by in situ concrete and waterproofed by PVC membrane.

Answer: There is at present no common standard measure of permissible leakage for tunnels. This is generally determined by the Engineer in conjunction with the owner on individual projects or standardized for a complete system. The two most important considerations should be how the tunnel will be used and how much it will cost to achieve the desired degree f water tightness. The condition of the ground and groundwater help to determine applicable control methods.

When permissible leakage is specified, it is usually given in two parts, a maximum flow for a given tunnel length or inside surface area, and a maximum leakage at any point.

Question 17: What are the Health and Safety Requirements need during the tunnelling process?

Answer: The health and safety requirements for tunneling are established by the agency having jurisdiction in your country. In the United States of America, those requirements are established by the Occupational Safety & Health Administration (OSHA) You can find more information at http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=10790#1926.800(k)(10)(i)

The International Tunneling Association (ITA-AITES) Working Group on Health and Safety has published several documents and they are at http://www.ita-aites.org/cms/164.html.

Question 18: Can you please provide some information about UDEC 2D modeling of forepoling supports.

Answer: My past experience with UDEC (2D discrete element code) is that it can model support elements, but treats each support as a continuum element in and out of plane. This may be problematic for modeling forepoling in a circular or arched tunnel as the loading on the forepoling support is nonuniform around the opening. Modeling a longitudinal section of the tunnel, with forepoling driven ahead of the face, and then showing excavation in incremental stages should give an indication of forepoling performance, but in essence the entry modeled is a flat-topped opening in and out of plane. Modeling a transverse tunnel cross-section only allows you to view support performance well after tunnel advance (> 2 diameters). This isn't so bad since it at least models loading about the entire periphery of the tunnel as it would be loaded in practice, and the supports are essentially continuous in and out of plane.

The preferred solution to this problem is to utilize a three-dimensional code, and since ground blocks are not allowed to fail in the successful tunnel support solution, a 3D FEM program may be suitable for analysis. Such a program would allow for the discrete assessment of included rock bolting in the system (if used), rather than treating them as infinite plates in and out of plane.

Question 19: What is the minimum allowable slope (Horizontal Grade %) and critical velocity of a tunnel to prevent scouring?

Answer: Not sure if your question relates to power tunnel or sewer tunnel hydraulics - in the world of sewer engineering, the velocity that sanitary engineers seek to maintain scour (and prevent sedimentation) is about 2 feet per second, but a velocity of about 3 fps may be desired to re-suspend solids that have already settled. From a tunnel hydraulics perspective, velocities are generally kept below 9 or 10 fps to prevent concrete scour or concrete plucking.

Question 20: What is the minimum overburden (height, m) of hard rock required for a self supporting tunnel?

Answer: At a portal, you can generally engineer a stable portal with 1/2 to as little as 1/3 of the diameter in massive rock. Along the alignment, this may depend on the other overburden conditions - for example, if you know its rock overburden, 1/2 a diameter can work. But there is soil as well, I'd be concerned about the waviness and characteristics of a top of rock contour that I cannot "see". In this case, 3/4 to a full diameter is more appropriate. In either case, you need to be prepared to install a nominal degree of support in the arch.

Question 21: What is the minimum spacing of rock bolts?

Answer: Rarely have I seen rock bolts spaced closer than 3 feet. In heavy squeezing ground, the tendency is toward longer bars rather than closer spacing.

Question 22: What is the minimum concrete lining of a tunnel? Example: for good ground and for fair ground?

Answer: In good rock, it becomes a matter of what can be installed within reasonable construction tolerances given the method of excavation, not what can theoretically do the job. For example, for a 10-ft diameter TBM-bored sewer tunnel, I recall setting 8 inches thickness as the minimum thickness from a pure constructability perspective. For a 43-ft wide D&B highway tunnel, we decided on 10 inches (for a lining that had no load carrying requirement) knowing that overbreak would generate a thicker lining that would serve to provide more of a tolerance desired for that size of opening. For fair rock, in addition to initial supports that will have had to have been installed already, 10 inches of concrete is probably ok, though a 12-inch thickness seems to find its way into more tunnels than is really warranted. Of course, clearances to initial supports may be a key factor, e.g. steel set inside flanges.

Question 23: I have a question about Jn (joint number) in Q system (NGI). If there are two sets of joint with same strike and opposite dip directions (the dip angles are 85 to 90 deg) in other word the sterionet shows these two sets at the margins with 180 degrees difference in dip direction. THen are they two sets or one set for choosing joint number (Jn)?

Answer: When determining the value of Q in the NGI Q System, remember that the parameter that is being evaluated when evaluating the joint number, Jn, is the Block Size which is a function of both RQD and Jn (i.e. Block Size = RQD/Jn). It is not unusual for rock masses have at least three sets of joints. If the bore hole was parallel to the two orthogonal joint sets, as you described in your question, then it is clear that the two joint set spacings and the RQD would be define the block size. Long story short, they should be considered two joint sets for choosing the joint number, Jn.


Question 1: When and what type of grouting is advocated in crushed phyllitic rocktype dominating rock mass conditions which otherwise exhibits poor bond strength with conventional portland cement. How is it the efficacy of the grouting be ascertained prior to the progress through drill and blast method of excavation

Answer: If the crushed phyllitic rock is the dominant rock mass along the reach of the proposed tunnel, perhaps a drill and blast excavation method is inappropriate and a soft ground excavation technique should be considered. If the crushed phyllitic rock is in a limited reach of the tunnel (such as an isolated shear zone) grouting can be considered. If the conventional cementatious grout is bonding poorly, it could be due to the rock absorbing water from the grout mix. Some ideas to consider are grouting with a higher water/cement ratio, pre-wetting the rock, or injecting silica gel prior to the injection of cement grout. Another option is to consider a chemical grouting program to strengthen the rock mass. Grouting is both an art and science, and we encourage you to contact a qualified specialist in this area. One technical reference that may be of value is "Practical Guide to Grouting of Underground Structures", by Raymond Henn, 1996, published by ASCE.

Question 2: Would you describe the pipe-jacking method through rock and soft soil and could you give some information about frictional resistance between pipe surface and outer soil mass?

Answer: General arrangements of pipe jacking and microtunnelling systems:

Pipe jacking, generally referred to in the smaller diameters as microtunnelling, is a technique for installing underground pipelines, ducts and culverts. Powerful hydraulic jacks are used to push specially designed pipes through the ground behind a shield at the same time as excavation is taking place within the shield. The method provides a flexible, structural, watertight, finished pipeline as the tunnel is excavated.

There is no theoretical limit to the length of individual pipejacks although practical engineering considerations and economics may impose restrictions. Drives of several hundred metres either in a straight line or to a radius are routine. A number of excavation systems are available including manual, mechanical and remote control. Pipes in the range 150mm to 3000mm, can be installed by employing the appropriate system. Construction tolerances are compatible with other tunnelling methods, and the pipe jacking method generally requires less overbreak than segmental tunnels, providing better ground support.

Excavation methods are similar to those employed in other forms of tunnelling using either manual or machine excavation. Shields, excavation and face support can be provided for a wide variety of ground conditions.

In order to install a pipeline using this technique, thrust and reception pits are constructed, usually at manhole positions. The dimension and construction of a thrust pit may vary according to the specific requirements of any drive with economics being a key factor. Mechanized excavation may require larger pits than hand excavated drives, although pipe jacking can be carried out from small shafts to meet special site circumstances.

Thrust Pit Set-up:

A thrust wall is constructed to provide a reaction against which to jack. In poor ground, piling or other special arrangements may have to be employed to increase the reaction capability of the thrust wall. Where there is insufficient depth to construct a normal thrust wall, for example through embankments, the jacking reaction has to be resisted by means of a structural framework constructed above ground level having adequate restraint provided by means of piles, ground anchors or other such methods for transferring horizontal loads.

High-pressure jacks driven by hydraulic power packs provide the substantial forces required for jacking concrete pipes. The ram diameter and stroke of the jack may vary according to an individual contractor's technique. Short stroke jacks with multiple spacer blocks, medium stroke jacks with shorter length pipes or long stroke jacks, which can push a full length pipe at one setting may be used.

To ensure that the jacking forces are distributed around the circumference of a pipe being jacked, a thrust ring is provided of a design dependent on the number of jacks being used. The jacks are interconnected hydraulically to ensure that the thrust from each is the same. The number of jacks used may vary because of the pipe size, the strength of the jacking pipes, the length to be installed and the anticipated frictional resistance.

A reception pit of sufficient size for removal of the jacking shield is normally required at the completed end of each drive. The initial alignment of the pipe jack is obtained by accurately positioning guide rails within the thrust pit on which the pipes are laid. To maintain accuracy of alignment during pipe jacking, it is necessary to use a steerable shield, which must be frequently checked for line and level from a fixed reference. For short or simple pipe jacks, these checks can be carried out using traditional surveying equipment. Rapid excavation and remote control techniques require sophisticated electronic guidance systems using a combination of lasers and screen based computer techniques.

Frictional resistance between the pipe and soil mass:

The frictional resistance between the pipe and soil mass (skin friction)will have an important influence on the design and construction of the pipe jacking system. Skin friction varies greatly depending on the nature of the soil mass the jacking will penetrate. A subsurface exploration program should be considered to identify skin friction values at your site. In some cases, water and/or air-jetting techniques can be considered to reduce skin friction.

For more information you can contact the Pipe Jacking Association at http://www.pipejacking.org/.

Question 3: I would like to know when tunnelling a vehicular tunnel and boulders are encountered on the roof of the tunnel what method would be used to overcome this dilemma?

Answer: The method of boulder removal would depend on the tunneling method being used which could depend on the strength or competence of the soil or rock being excavated (i.e. the matrix in which the boulder is embedded.) If the matrix it relatively soft/weak, the tunnel will probably be advanced using a TBM or road-header. In either case the boulder can be removed and the resulting void backfilled with either shotcrete or CIP concrete. If the matrix is relatively hard/strong, it would probably be practical to leave the boulder in place and remove only that portion of the boulder that intrudes into the tunnel cross-section, by either drill and blast or mechanical chipping methods.

Question 4: What precautions need to be taken to minimize structural settlement when driving a 2-m diameter microtunnel 20-m below ground level (GL) under bridges having: 1) piled foundations with toe of piles 10-m below GL and 2) pad foundations 2.5-m below GL? Soil is sandy with clay particles and some gravel, water table is 1-2-m below GL

Answer: In soft ground tunneling, particularly below the water table, structural settlement is a big concern. Structural settlement can occur due to two effects: water table depression resulting in higher effective stresses and subsequent consolidation (cohesive soils) or settlement (granular soils); and subsidence due to ground loss associated with the tunneling operation.

Water Table Depression - The settlement depends on the compressibility of the soil and is usually very small in granular soils unless very loose. If you are not planning to dewater prior to tunnel construction, this is not an issue.

Ground Loss - Results in a "settlement trough" which can be characterized based on the diameter of the tunnel, depth of the tunnel and the soil type. There are several methods available to predict the size and shape of a settlement trough (e.g. Tunnel Engineering Handbook, Bickel & Kuesel). Once the settlement trough has been characterized, an assessment can be made as to whether such settlement will damage surface structures.

There are three forms of ground loss in soft ground tunneling: face losses, shield losses and tail losses. Methods of limiting lost ground include:
  1. Full and proper face control at all times, especially while shoving the shield.
  2. Limiting the length-diameter ratio for the shield, making directional control easier and reducing the effects of pitch and yaw.
  3. Rapid installation of ground support.
  4. Rapid expansion , pea-gravelling, and/or contact grouting of ground support.

In special cases, other steps may include:

  1. Use of compressed air.
  2. Consolidation grouting of the ground before tunneling.
  3. Consolidation grouting from the tunnel face.
  4. Compaction grouting between the tunnel and foundations.
  5. Underpinning structures by any of a group of methods (e.g. micropiles).
  6. Use of protective walls, including slurry walls or soil-cement structural walls embedded below the tunnel.

Question 5: What are the methods or procedures for chemical pressure grouting of tunnel lining joints along waterstop area to seal water from coming in?

Answer: Chemical grouting to seal against water that is penetrating joints in the lining can be a complex procedure. The actual rate of water infiltration is affected by the local water table, local geology, tunnel formation drainage, and the integrity of the joint/lining. These factors will control the type of chemical grout used, location of grout tubes, and actual grouting pressures.

We suggest you contact a specialty contractor experienced in chemical grouting of underground structures. One textbook you may find of interest is "Practical Guide to Grouting of Underground Structures", by Raymond W. Henn, 1996, published by ASCE.

Question 6: Q system is a well-known system to classify rock mass in which its logarithmic scale provides a initial guide for primary support design. Could you please tell me how to explain if a intersection is on the top left corner which is not in any provided zones (from 1-9). For example if values of the index Q=0.01 and Equivalent Dimension(De)=70 or Q=200 and De=88. These two intersections are located in different rock mass quality on the scale. What kinds of support should be used?

Answer: Norwegian Geotechnical Institute (NGI) has developed the Q-method since the 1970s. The Q-method is a numerical description of the rock mass quality with respect to tunnel stability. The Q-value is defined by a function consisting of six parameters which may be estimated either from geological mapping or from in situ measurements and from drill cores. The Q-method is used internationally for general description of rock mass quality, and as a guide for estimating tunnel support requirements.

The main purpose of this project has been to provide practical advice for the systematic collection of engineering geological data during site investigation. The Q-method has been used as a base for the presentation of data of uniform format. Data from different sources can easily be compared in this way, and a better evaluation of the engineering geological condition can be carried out.

A manual with a description of the practical use of the method has not been available. The aim of the present project has been to produce manuals for the use of the Q-method in different ways. The basic report "Practical use of the Q-method" describes how the Q-method can be used for estimation of rock support. Other reports deal with the use during field mapping, core logging and in TBM-tunnels.

These publications are available from the NGI through their web site: http://www.ngi.no/english/
  • "Practical use of the Q-method", Report No. 592046-4
  • "Use of Q-system in Weak Rock Masses", Report No. 592048-1

Question 7: Rock bolts are commonly used in rock tunneling. My question is what are important influential factors used to decide when untensioned dowels or tensioned rock bolts should be applied? Does numbers of joints in rock, in situ stress and rock strength have an effect?

Answer: Intact rock needs no support. It is the breaking away of rock at joints that causes rockfall. Where the rock is not to badly fractured, rock bolts are use to hold the rock together so that it will behave as intact rock. Rock bolts can be used to either support the rock until a permanent support system can be constructed, such as a reinforced concrete liner, or to permanently reinforce the rock by increasing the shear resistance in the joints.

Tensioned rock bolts can serve either of these purposes. Bolts used for permanent support must be protected from corrosion.

Untensioned dowels will not increase joint shear resistance and so are not used for rock reinforcement but can only hold rock blocks in place. Dowels are sometimes used in shear by installing them across the joint plane to hold a rock block in place.

Question 8: Are you aware of any statistical method to estimate the number of boulders that might be encountered in a soil tunnel based on findings of the geotechnical

Answer: The short answer is, no, we are not aware of any statistical method for estimating the number, volume or occurrence of boulders in a soft ground matrix. Characterizing the presence of boulders in a soil matrix is a problem that is not unique to tunnel engineering, but is a common bane of geotechnical engineering. The best defense is to conduct adequate geological and geotechnical investigations. As you know, because of the manner in which we advance geotechnical borings the presence of boulders often goes undetected. A complete understanding of the geological setting of the tunnel site will often give indications of the potential for the presence of boulders.

Probably the best way to characterize the presence of boulders along a tunnel alignment is to construct a large diameter pilot bore so that geologic mapping and testing can be conducted.

Question 9: During the construction of a tunnel one wants to find what type of rocks are there subsurface. What are the geophysical methods to find the rock type? How one can find out the weak zone and fault zone in the rocks for tunnel designing? Please briefly discuss the various geophysical methods.

Depending on tunneling depth, rock/soil types, stratigraphy and depositional setting, anomalous ground structures/features, groundwater, etc., a variety of geophysical methods may be available to preliminarily characterize subsurface conditions overlying/underlying proposed tunnel alignments. As with all geophysical surveys, invasive drilling/sampling programs are typically required to confirm survey results and to provide sufficient ground mass engineering data to effect proper tunnel designs and to select appropriate excavation methods. Specific information pertaining to suitable geophysical techniques and methods may be found within the readily accessible on-line manual entitled "Application of Geophysical Methods to Highway Related Problems", located at http://www.cflhd.gov/geoTechnical/.

Question 10: What is the minimum overburden of hard rock required for a self supporting tunnel.

The minimum thickness of rock overburden for a "self-supporting" tunnel is dependent on many variables related to (1) ground strength, (2) applied loads, (3) opening geometrics, and (4) time. Strength variables include both the strength of the intact rock (the solid rock portion) and rock mass (bulk strength of the rock mass, including discontuities - generally much less than the intact strength). Strength is further defined by such things as bedding/foliation, weathering characteristics, and groundwater conditions. Applied loads include overburden loads, in situ tectonic stresses (often in the form of high horizontal stresses), hydrostatic pressures, variable loads due to nearby, active excavations, seismic loads, and possibly surface surcharge loads (for shallow tunneling applications). The geometry of the opening (size, shape, and span dimensions), as well as its alignment relative to loading and the geologic setting (orientation and dip), greatly defines self-supporting performance. And, finally, time must be accounted for when evaluating stand-times for specified unsupported spans.

Self-supporting tunnel designs require intimate knowledge of the rock mass to determine maximum unsupported span capacity, as well as the stand-time of a given span dimension for the intended use and environment of the tunnel. The amount of rock cover required, therefore, can be highly variable from one setting to the next. For example, a shallow tunnel driven through a massive granite (with few discontinuities) may be able to stand unsupported for decades (centuries?) with spans extending upwards of 10-20m or more. In comparison, a tunnel driven in weak, highly bedded shales and sandstones at greater depths, and subject to high lateral tectonic loads, may only tolerate unsupported spans of a few meters, if at all.

In addition to simply being able to drive the intended tunnel dimension as an unsupported tunnel, other factors may influence the minimum overburden requirements. For instance, rock overburdens of sufficient thickness to tolerate caving without causing surface subsidence (piping), should the tunnel collapse, may be required to protect surface structures. In other cases, rock interburdens may be needed to separate tunneling from overlying aquifers - to both mitigate water in the tunnel, as well as the risk of capturing or contaminating the aquifer.

Question 11: How the stress at different location can be estimated if we have the values for insitu stress at some nearby location?

The insitu stress is generally composed of virgin stress (governed by Poisson's ratio) and tectonic stress. The virgin stress ratio can be simply calculated using Poisson's ratio such that horizontal stress/vertical stress = v/(1-v), where v = Poisson's ratio. Therefore, if we have a measured in-situ stress ratio at one point and know the Poisson's ratio, then the tectonic stress at the point can be calculated as tectonic horizontal stress = measured horizontal stress - virgin horizontal stress. Assuming the identical tectonic stress has been applied to a nearby location, the initial stress at the point can be estimated. .

Question 12: How the measured 3d stress can be converted into horizontal and vertical components for that location?

If the measured 3d stresses are principal stresses, then the horizontal and vertical stress can be calculated using the tensor transform theory.

Question 13: How the supports are designed for the tunnel if we know the insitu stress values at that location.

There are three types of loadings to design tunnels: loosening load, excavation load and overstress load. What load should be used to design tunnel is related to tunnel loading and failure mechanism in different depth. In shallow depth where the insitu stress is low, the loosening load governs and therefore wedge instability is main tunnel failure mechanism. Therefore, tunnel support in a shallow depth is designed for the loosening loads not the initial stress. For a tunnel in deep depth, the wedge stability becomes less problematic due to high confining stress, instead, stress induced failure becomes a governing factor. Stress induced failure potential is estimated by comparing rock mass strength and amplified insitu stress due to tunnel excavation. For the required support pressure calculation, the interaction curve can be used. Once the in-situ stress and rock mass properties are known, one can develop tunnel interaction curve (Figure 1) using FEM or analytical solution and the required support stiffness can be estimated as shown in Figure 2.

Figure 1
Figure 1 as discussed above

Figure 2
Figure 2 as discussed above

Systems/Fire Safety

Question 1: We are a small engineering consultant firm working on specifying the ventilation requirements for a tunnel underneath a airport runway. The length is approx 800 feet and the traffic pattern is assumed to be only that traffic on the airport property to move luggage, fuel,misc. NFPA 502 provides minimum guidelines for a tunnel fire. My question is what are the normal ventilation guidelines,if any, for tunnel design?.

Answer: The ventilation of a tunnel for normal operations has the main objective of diluting the concentration of CO to acceptable limits for the traveling public inside a tunnel. FHWA and EPA have suggested the following limits based on the exposure time inside the tunnel.

max 120 ppm for 15 minutes
max 65 ppm for 30 minutes
max 45 ppm for 45 minutes
max 35 ppm for 60 minutes

In addition, during and emergency inside a tunnel, the ventilation system could be used to help manage the heat and smoke of a fire inside the tunnel.

Question 2: Is there a minimum average airflow while developing a tunnel? Phrased differently. Is there a minimum retention time of an airbourne contaminate in a developing tunnel?

Answer: Standard tunnel design references address vehicle emission concentration limits and design procedures for limiting vehicle emissions in operating tunnels.

For limiting contaminants during tunnel construction, OSHA has air quality worker health and safety requirements that would address a tunnel under construction.

Question 3: Is an Automatic Fire Sprinkler system recommended to be provided for Fire Protection in a transportation tunnel?

Answer: There are very few tunnels throughout the world that have sprinkler systems installed in them. Given our present knowledge of the subject the use of sprinkler systems in road tunnels are not recommended for the following reasons:

  • Water can disperse burning liquids over a large surface area;
  • With some materials, contact with water can produce dangerous reactions;
  • The steam which is produced can reduce visibility;
  • The efficiency is low for fire inside vehicles;
  • Even if the flames are extinguished, the metal in the vehicle does not cool quickly and inflammable products can continue to give off gases leading to the presence of explosive mixtures;
  • Hot surfaces may ignite vaporizing petrol or LPG gas;
  • The water (or foam) that is distributed may, as in the case of a petrol tanker fire, be insufficient to extinguish the fire on a vehicle carrying a large quantity of fuel;
  • The smoke layer is cooled down and de-stratified, so that it may cover the whole tunnel;
  • As a consequence, sprinklers must not be used in the fire area before all people have been evacuated.

The main reason why sprinklers are not used is because they have not proven to save lives when used to extinguish and/or control a tunnel fire. The only benefit presently know is that these systems may be effective in cooling down the area around the fire, so that fire fighting can be more effective and the risk of the fire spreading to other vehicles is reduced.

The Japanese have more than 6000 road tunnels and are the biggest users of sprinkler systems in tunnels (82 tunnels as of 1999). Sprinklers are required in all tunnels greater than 10000 m and in shorter tunnels (longer than 3000 m) if heavy traffic. However, they experience the same issues as listed above.

Ultimately, the ventilation system is used to control the heat and smoke within the tunnel to protect the people within the tunnel until the emergency response personnel arrive to extinguish fire. Recent testing that has been done in the United States, specifically the Memorial Tunnel Fire Ventilation Test Program, has performed 100 full scale tunnel fires to assist tunnel engineers and operators to configure a tunnel's ventilation system to do just that. Information from the above mentioned testing program and the computational fluid dynamics model created from these tests can be found at the following website http://www.tunnelfire.com/. Also, to find out more on what other countries viewpoint is on the use of sprinklers please refer to "Fire and Smoke Control in Road Tunnels", PIARC - World Road Association, 1999.

Question 4: Is there any 'UL' listing or certification for Jet Tunnel Fans like we have for Power Ventilators (ex: UL 705, UL 762), if yes, give details. Also, please advise us the equal BS standards for the same.

Answer: Jet fans used in tunnel applications are generally custom specified and there is no UL certification requirement for the total fan unit as a final assembly.

However, because jet fans are, in most cases, considered part of the tunnel life-safety system, specifications must be rigid in regard to high temperature survivability of the motor, motor power junction box and the rotor/blade assembly from an in-tunnel fire. All paint products and attenuator insulation should be specified for low-smoke, toxicity, flammability and flame spread properties (UL 723/NFPA 255/ASTM E84 compliance recommended).

Jet fan motors are usually rated at 480v/3 phase and range in duty from 20 to 75 hp which is well above the duties of the commercial type roof/wall ventilators typically certified under the UL Standards referenced by the inquirer. Jet fan motor conditions such as vibration and over temperature are usually monitored continuously. Motors, monitoring devices, power and control wiring is all specified to applicable ANSI, IEEE and NEMA (Design B) standards.

Question 5: What are the mandates for fire fighting equipment and personnel in transportation tunnels

Answer: I am not aware of any mandates from the Federal Highway Administration on the subject. However, the National Fire Protection Association (NFPA) has published numerous standards, codes, recommended practices, and guides for fire and safety issues. I would refer you to NFPA 502 entitled Standard for Road Tunnels, Bridges and Other Limited Access Highways 2001 Edition. You may find this publication on the NFPA website located at: http://www.nfpa.org/.

Question 6: I would like to know how to evaluate wind loads in tunnels, and the effect of the cars. The cars also generated a wind load inside of the tunnel. We are placing PLADUR laminae and I need to calculate the effect of the air generated by a vehicle in the laminae.

Answer: We are not sure if we understand the thrust of your question. We are interpreting it as a question about the structural design of tunnel linings. If we have misunderstood, please restate your question and send it to us again.

Evaluation of wind loads, due to wind flowing into the tunnel or air turbulence created by vehicles passing through the tunnel, is not considered in the structural design of a tunnel. Forces due to these conditions undoubtedly exist, but their magnitude is quite small in comparison to the magnitude of loads such as soil overburden, earth pressure, self-weight of the tunnel structure, vehicle load, and others that are evaluated during the design of tunnel lining structures. As a result, design codes, such as AASHTO, do not address the application of this force in the design of tunnel linings.

Question 7: Seeing that most fires start from overheating of the electrical systems/devices and wiring anyway, wouldn't it be logical to have sprinklers mandatory in the service tunnel seeing as the service tunnel is also utilized as emergency egress/exit? Or are we misusing the objective of having a service tunnel in the first place?

Answer: When you talk about service tunnels you need to determine what is their primary use/purpose, utilities or emergency egress. In my opinion, your are defeating the purpose of an emergency egress if you are filling them up with utilities that could themselves, as you mentioned in your question, be part of the hazard. It is often to easy to locate utilities in locations that are not necessarily the best location when looking at the overall picture but the initial cheapest cost to install them often governs. Also, to comment on another statement you made, fuel supply piping in an emergency egress is not a good idea because of their potential combustibility or adding fuel to the fire, which in return eliminates your mode of emergency egress.

As far as mandating the use of sprinkler systems in service tunnels so they can be used as an emergency egress, this would not help an individual trying to use the emergency egress as a mode of escape. The reasons for this is that water sprinkler systems would render the tunnel unusable because visibility will be very little due to the amount of water used to put out the fire. Also, the steam generated when the water hits the fire further limits visibility. If a foam system were used it will be even less usable as a mode of emergency egress.

The bottom line is really dependant upon the very simple question I asked in the beginning - what is the primary use of the tunnel? If the tunnel owner needs to install many utilities it would be appropriate to provide one access tunnel for utilities and an emergency egress tunnel. Granted, this will add cost to the project but in the long run it will be much better for emergency purposes.

Question 8: Are there any national guidelines or recommendations on the color of an egress door in a tunnel? (i.e., requirements for it to contrast with surroundings and be visible in an emergency situation.).

Answer: All doors used for cross passages and egress in road tunnels should meet the requirements of the National Fire Protection Association (NFPA). In specific you should consult Standards NFPA 502, NFPA 101, and NFPA 80.

The current practice is to use stainless steel doors meeting the above NFPA standards. Stainless steel doors inside a tunnel will resist corrosion and the harsh tunnel environment much better than a painted door. Your local fire department should approve the use of painted doors.

Question 9: An existing 300' tunnel on an airfield under a runway is designated "NO FUEL TRUCKS". The origination of the restriction is unclear. I don't find this restriction in NFPA 502 or FAA Circulars...any help? Are you aware of tunnels where foam systems have been install to allow the transport of hazardous materials?

Answer: Restriction of truck traffic through a tunnel is at the discretion of the tunnel owner and the local fire fighting agency or agencies. As stated below, you will not find it in the NFPA 502 or FAA circulars for that reason.

There are several tunnels throughout the world in which a fire suppression systems (either water, foam or both) have been installed to handle a fire incident; however, there is much debate on their effectiveness in putting out a fire as well as the protection of the individuals in the tunnel when these systems are used. The Japanese have the most experience in using fire suppression systems than any other country. There have been a few in the United States; however, one of them still does not permit hazardous cargo through because the local fire department determined the use of a fire suppression system does not support the allowance of hazardous cargo through the tunnel.

On a positive note, there is an international initiative that is developing a risk model that examines the risk of hazardous cargo through tunnels versus designated detours. The effort is trying to establish which route has the most risk transporting hazardous cargo. We hope that this will provide owners with information that will allow them to determine whether the restriction of hazardous cargo through a particular tunnel is appropriate. I do not know when the above initiative will be complete but it might help provide justification for opening tunnels up to more truck traffic.

Question 10: When designing the emergency egress (stairway) from the underground station, is there any code requiring that the egress has to be ventilated? The emergency egress is not located in the station, but further away in the tunnel. NFPA 130 and NFPA 502 do not deal directly with this issue.

Answer: The staircase (or stairwell) should be positively pressurized in accordance with NFPA 92A, which is referenced in NFPA 130 via NFPA 101 (ie 130 points to 101 which points to 92A). Basically a positively pressurized stairwell provides a tenable environment for passengers to escape.

In the example discussed below, changing the fan to a supply fan and have a riser the full height of the staircase to evenly distribute the air flow would be a solution. This was proposed for the emergency egress stairwells on East Side Access in New York.

If there is an active tunnel ventilation system it is often the case that the tunnel ventilation system is capable of keeping the egress stairwell clear of smoke - but this should be demonstrated by ventilation modeling.

Question 11: Recently I came across an article on the DoT web site entitled "Prevention and Control of Highway Tunnel Fires" which spelled out investigation of tunnel fires in a number of cases comprehensively. Whilst this article pinpointed aspects on monitoring and ventilation arrangement, I should be grateful if you would enlighten me whether there are recent articles/researches in USA on adequacy of tunnel structures (especially reinforced concrete structures) under fires. I understand that some of the European countries had done researches on this aspect. It would be most beneficial if we could also gain reference from experience of the USA in this regard

Answer Recently, in the USA, we have completed a very comprehensive test program on tunnel fires. The test, The Memorial Tunnel Fire Ventilation Test Program (MTFVTP) consists of a series of full-scale tests that were conducted in an abandoned road tunnel, the Memorial Tunnel in West Virginia. Various tunnel ventilation systems and configurations of such systems were operated to evaluate their respective smoke and temperature management capabilities. These tests generated a significant database relevant to the design and operation of road tunnel ventilation systems under fire emergency conditions.

The test portion of the Project consisted of three separate phases, which are completed and documented. Additional information on the program and how to obtain a copy of the CD-ROM is at the Internet address http://www.tunnelfire.com/ where sets can be ordered.

Question 12: I am the designated representative with IBTTA for Attikes Diadromes SA, a new IBTTA member organization and I am also a member of the Freeway Operations Committee of the TRB. I am heading the Operating Agency of the Athens, Greece, Toll Ring Road called the "Attiki Odos Motorway" or the "Attica Tollway". The Tollway is a 65 km long toll highway forming the newly built Athens Peripherique. The Tollway comprises from three lanes in each direction and its linked with the major urban network of the Athens Metropolitan area through 24 Grade Separated Interchanges. There are 38 toll stations, all of them installed at the entry points to the Motorway, so that entering traffic pays toll only once. In total 195 toll lanes are in operation providing both manual, as well as ETC toll collection abilities, through a modern toll system.

The Tollway crosses major arterial roads in the Athens area and it has been built with closely spaced interchanges in the central section with distances of 1 to 2 km between them, while parts of it are running along the foothills of a mountain. Out of the 130km of carriageway (length of both directions), 16km are tunnels and cut and cover sections.

We are following NFPA 502 code for all elements of Fire Protection (design, construction, operation). We understand that a major revision to the 1992 standards has been undertaken by the current 1998 standards, which are the ones used for the Tollway, which was under design at the time. The European standards issued recently, following the Mont Planc accident, are making the distinction between existing and new tunnels, calling existing tunnels all tunnels whose design has been approved. We wonder how NFPA deals with existing tunnels and what NFPA calls existing tunnels. If there is nothing that one can do to construct escape routes by providing points of exit every 300m or 500m (as the case may be) can it be a substitution by beefing up operational and monitoring measures?

Answer You mentioned that out of 130Km of carriageway (length of both directions) 16Km are tunnels and cut and cover sections. And that NFPA 502 1992 and 1998 were used during the design of The Attica Tollway.

You posed several questions and here are our responses to each of your questions.

  1. We wonder how NFPA deals with existing tunnels and what NFPA calls existing tunnels.

    Based on Section 1-5.3 of NFPA 502, 1998 an existing tunnel includes those under operation and facilities, equipment, structures, or installations that were approved for construction or installation prior to the effective date of the standard.

    For existing tunnels, the portion of the standard that cover emergency procedures shall be applied.

  2. If there is nothing that one can do to construct escape routes by providing points of exist every 300m or 500m (as the case may be) can it be a substitution by beefing up operational and monitoring measures.

    If there is nothing you can do to provide points of exits every 300 m or 500 m (as the case may be) a waiver from the authority having jurisdiction shall be obtained. In addition a complete Emergency Response Plan should be developed and must be approved by the authority having jurisdiction. As an option for those tunnels that are divided by a minimum of 3 hours fire-rated construction or where tunnels are in twin bores, crosspasages between the tunnels might be permitted to be utilized in lieu of emergency exits.

    Additionally, we would like to inform you that since 1998, the NFPA 502 has been updated twice, in 2001 and just recently on September 2004.

Question 13: What is the emergency response while designing a road tunnel system?

Answer: The agency that is responsible for the safe and efficient operation of the tunnel should anticipate and prepare a plan for emergencies that could happen inside the tunnel. The emergency response plan should be developed with the assistance of other appropriate agencies. More details and guidelines can be found in the NFPA 502 Standard for Road Tunnels, Bridges, and Other Limited Access Highways, published by the National Fire Protection Association, Quincy, MA.


Question 1: How can exhaust ports, in a smoke extraction system, be designed to fully open when subjected to the heat of a fire, drawing smoke and hot gases into the exhaust duct?.

Answer: I need to make an assumption about the intent of your question. The scenario you are describing probably relates to the exhaust ports, that are in a suspended ceiling, over the roadway, in a tunnel.

The objective is to create a larger port opening to increase the quantity of smoke exhausted in the vicinity of the fire. If this can be done with certainty then it may be possible to reduce the size of the ventilation plant.

Several methods for increasing the port area during a fire have been discussed among tunnel engineers:

  1. Spring loaded dampers with a fusible link, similar to a fire damper used in a common building HVAC duct.
  2. Ceiling panels that will melt away due to the heat of a large fire.
  3. Motor operated dampers.

The primary reasons these are not installed in road tunnels are the following:

  1. Heat carried away from the fire could cause the wrong ports to open. This would interrupt the planned emergency airflow.
  2. Corrosive atmosphere may cause the damper parts to fail over time and not respond when necessary.
  3. High cost for procurement, installation, maintenance and repair.
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Updated: 10/19/2015
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